This invention relates to surgical alteration of the cornea of the eye.
The cornea, the transparent dome-shaped anterior portion of the fibrous covering of the eye, comprises several layers. The anterior surface includes the corneal epithelium, consisting of several layers of cells covering the front of the cornea. The posterior surface includes a corneal endothelium, consisting of a layer of cells. Between the corneal epithelium and the corneal endothelium is the corneal stroma, or substantia propria corneae, composed of about 60 superimposed connective tissue lamellae held together in placed by an interstitial cement substance and separated in other places by corneal spaces containing corneal corpuscles. The stroma is bounded anteriorly by the anterior lamina, also termed Bowman's membrane, consisting of closely interwoven fibrils, and posteriorly by the posterior elastic lamina, also termed the membrane of Descemet. In the average human eye the cornea is a part-spherical body about 0.5 mm thick over most of its extent; the stroma itself comprises about 90% of the total corneal thickness.
The cornea contributes a majority of the refractive power of the human eye, and for this reason surgical alteration of the shape of the cornea, often termed "keratoplasty", can result in significant changes in the refractive characteristic of the eye. Techniques of refractive keratoplasty can be classified as keratotomy, by which the curvature of the cornea is altered generally and the thickness is not substantially altered; or as lamellar refractive keratoplasty, by which the curvature of only the anterior surface of the cornea is altered, and the cornea is made thicker or thinner over some of its extent.
Known techniques of lamellar keratoplasty in which the anterior curvature of the cornea is altered include the classic Barraquer technique, in which a portion of the cornea is removed, reshaped and surgically replaced. In this technique a lamellar corneal disc, comprising the anterior surface and some stromal matter, is removed from an eye and positioned on a plastic holder with one surface of the corneal disc against a part-spherical cavity in the holder; then the corneal disc is frozen in the cavity and a part-spherical surface is cut from the other surface of the corneal disc using a cryolathe; and finally the resulting lenticule is thawed, replaced on the cornea and sutured in place. Where the part-spherical surface cut from the corneal disc has a raduis of curvature greater than that of the cavity in the plastic holder, the resulting lenticule is thicker near its center than near its margin; when such a lenticule is grafted onto a cornea of a hyperopic eye, it imparts a steeper curvature to the anterior surface, correcting the hyperopia. Conversely, where the part-spherical surface cut from the corneal disc has a raduis of curvature less than that of the cavity in the plastic holder, the resulting lenticule is thinner near its center than near its margin; when such a lenticule is grafted onto a cornea of a myopic eye, it imparts a flatter curvature to the anterior surface, correcting the myopia.
C. A. Swinger et al., 1986, Jour. Refractive Surgery, Vol. 2, pp. 17-24, describes a variation of the classic Barraquer technique, in which the epithelial surface of the corneal disc is placed onto the base upon a die surface whose curvature is selected according to the type and degree of correction required. Material is then removed from the corneal disc by making a planar cut through the disc using a microkeratome, and the resulting lenticule is replaced on the cornea and sutured in place.
T. Krwawicz, 1964, Am. Jour. Ophthalmol., Vol. 57, pp. 828-33, describes altering corneal curvature for correcting human myopia by a technique in which a thin lamella of the stroma is removed from the central part of the cornea for changing its curvature to modify the refraction of the eye. Krwawicz's technique consists of making an anterior incision into the cornea near the margin of a circular operating area, not going beyond the superficial layers of the stroma; splitting the stroma in two planes parallel to the anterior and posterior surfaces; and excising the resulting lamellar flap with a punch forceps.